The present invention concerns a process and device for catalytic cracking of a hydrocarbon charge in a descending bed, utilizing an improved contact zone between the charge and the catalyst.
It is known that, in the petroleum industry, "Fluid Catalytic Cracking", or "FCC," has come to occupy an increasingly important place in refining, since it allows the composition of crude oils to be adjusted to respond to the demand of the refined products market.
In these processes, the charge is cracked in the gas phase in the absence of hydrogen. The reaction temperature is about 500.degree. C. and pressure generally approaches atmospheric pressure. During the cracking reaction, the catalyst becomes covered with coke and traces of heavy hydrocarbons, and the heat generated from the combustion of this coke during the regeneration operation in the presence of air or oxygen makes it possible to heat the catalyst to the desired temperature in order to supply the necessary calories to the cracking reaction after the catalyst has been reinjected into the reactor.
These FCC processes are habitually carried out in ascending flux reactors, thus giving the English-derived term of "riser reactor." However, this method of operation poses a number of problems: the catalyst particles in the fluidized bed exist in an unstable equilibrium, since they tend, first, to rise because of the ascent of the gases which ensure fluidized bed sintering and vaporization of the charge, and second, to fall because of the weight thereof.
As a result, the C/O ratio between the catalyst flow rate C and the flow rate O of the charge to be processed is limited to a maximum value normally of between 3 and 7 in current reactors, and usually of about 5.
Furthermore, in ascending flux reactors particles accumulate near the reactor walls, thus producing an excess cracking of the hydrocarbons in these locations, a phenomenon which forms coke, hydrogen, methane, and ethane instead of the products having the desired high octane number, while, in the center of the reactor, where fewer particles are found, insufficient conversion of the charge occurs.
Finally, while, overall, the catalyst grains rise in the reactor, some of them may fall back in certain places in proximity to the wall, because of the accumulation phenomenon explained above. This occurrence, known in English as "back-mixing," also leads to a localized reduction of conversion, since the grains which fall back are partially deactivated and produce less effect on the charge than the grains which rise. This phenomenon is especially troublesome because the aforementioned C/O ratio drops.
In order to remedy the problems posed by the riser, it was long ago suggested that reactors exhibiting downward catalyst flow, or "downers" be used (see, in this regard, U.S. Pat. No. 2,420,558).
In fact, it is known that the basic difference between these two types of reactors lies in the fact that the relative positions of the catalyst and of the charge remain substantially the same along the entirety of the downer, since the vapor and solid phases are placed in motion by the effect of gravity.
Accordingly, there is an absence of back-mixing, the radial homogeneity of the catalyst in the reactor is preserved, and the flow in this reactor is of the piston type. This makes it possible to impart good selectivity to the cracking reaction.
Furthermore, reaction times can apparently be reduced substantially as compared with the riser and may be substantially shorter than 1 second, and it appears possible to increase freely the flow rate of the catalyst with no adverse effect on particle movement, as is the case in a riser.
However, the use of a downer poses many difficulties, with the result that no one has as yet truly risked conversion from an ascending to a descending flow on an industrial scale.
Indeed, while the downer is supposed to allow very short reaction times, it is technically very difficult to produce the mixture and achieve vaporization and separation of the hydrocarbons from the catalyst grains, when these operations must be carried out in a fraction of a second and with flow rates approaching 1,500 tons per hour of catalyst and 300 tons per hour of hydrocarbons at a high boiling point.
In particular, the downer exhibits a disadvantage linked to the initial mixture of catalyst and charge. In fact, the catalyst tends to fall immediately without producing back-flow or recirculation, thus causing an adverse effect on the initial transfer of mass and heat with the charge.
If the inlet flows of catalyst and charge were perfectly uniform, this effect would be insignificant. This is not the case, however, and, for this reason, in a cracking reactor the solids/gas mixture consists of an alternation of catalyst-rich and catalyst-poor areas.
In a downer, there is no mechanism allowing the charge to travel from one area to another. Accordingly, the fraction of hydrocarbons in contact with an area of low solids density will persist along the entire length of the reactor and will undergo inadequate thermal cracking caused by a premature deactivation of the catalyst. On the other hand, the hydrocarbons in a zone of high solids density may undergo excessive cracking.
To optimize simultaneously the charge/catalyst mixture and the quality of the cracking reactions themselves, U.S. Pat. No. 5,458,369 proposes a device in which the charge is pulverized, placed in contact with the catalyst, then partially cracked using an ascending flow. Next, the direction of flow is reversed and cracking is completed using a descending flow.
Nevertheless, this device is difficult to manufacture from a mechanical standpoint, and does not permit a very high-performing mixture in the case of high catalyst flow rates. In fact, when the direction of flow of the catalyst/charge mixture is reversed, the catalyst tends to become pelletized near the walls of the apparatus and is thus insulated from the vaporized charge.